1. Field of the Invention
The present invention relates to a feedforward amplifier for carrying out low distortion amplification in a radio frequency band.
2. Description of Related Art
A feedforward amplifier that achieves a low distortion characteristic by feedforward distortion compensation is often used as an amplifier for carrying out low distortion amplification in a radio frequency band such as VHF, UHF and microwave frequency bands. The feedforward distortion compensation can implement favorable distortion compensation in principle, and has an advantage of being able to configure a very low distortion, small amplifier. However, it has a problem in that when the characteristic of the amplifier varies because of ambient temperature or deterioration with age, its distortion compensation range is reduced and the distortion characteristic is impaired significantly.
To solve the problem, a method is proposed that injects a pilot signal into a loop constituting the feedforward distortion compensation system, and controls the amplifier or the loop constituting the feedforward system by detecting the pilot signal.
FIG. 1 is a block diagram showing a configuration of a feedforward amplifier disclosed in Japanese patent application publication No. 7-77330. The technique is an example that injects the pilot signal into the feedforward distortion compensation system to control the feedforward system.
In FIG. 1, the reference numeral 1 designates an input terminal of the amplifier; 2 designates a splitter for distributing an input signal to two paths; 3 designates a first vector regulator for electrically regulating the amplitude and phase of a signal passing through the first path; 4 designates a main amplifier for amplifying the input signal; 5 designates a delay circuit for delaying the input signal distributed to the second path by the splitter 2; 6 designates a splitter/combiner for distributing a part of the output signal of the main amplifier 4 and for combining the distributed output signal with a part of the input signal passing through the delay circuit 5; 7 designates a directional coupler; and 8 designates a pilot signal generator.
The reference numeral 101 designates a distortion detecting loop that includes the splitter 2, first vector regulator 3, main amplifier 4, delay circuit 5 and splitter/combiner 6, and cancels the input signal component by combining the input signal with the output of the main amplifier 4, thereby extracting a distortion component generated by the main amplifier 4. Here, the pilot signal supplied from the pilot signal generator 8 is injected into the output of the main amplifier 4 via the directional coupler 7. The pilot signal is used for controlling a distortion canceling loop 102 as described later.
The reference numeral 9 designates a delay circuit; 10 designates a combiner; 11 designates a second vector regulator; 12 designates an auxiliary amplifier; 13 designates a directional coupler; 102 designates the distortion canceling loop including the delay circuit 9, combiner 10, second vector regulator 11, auxiliary amplifier 12 and directional coupler 13. The reference numeral 14 designates a directional coupler; 15 designates an output terminal of the amplifier; 16 designates a level detector; 17 designates a pilot signal detector; and 18 designates a controller for controlling the first vector regulator 3 and the second vector regulator 11.
Next, the operation of the conventional feedforward amplifier will be described.
The output signal of the main amplifier 4 passes through the splitter/combiner-6, and its major part passing through the delay circuit 9 is supplied to a first input terminal of the combiner 10 installed on the output side. The distortion component extracted by the distortion detecting loop 101 appears at a terminal of the splitter/combiner 6, passes through the second vector regulator 11, is amplified by the auxiliary amplifier 12, and is input to the second input terminal of the combiner 10. The combiner 10 combines the output signal passing through the delay circuit 9 with the distortion component amplified by the auxiliary amplifier 12 in the same amplitude but in the opposite phase, thereby canceling the distortion component and producing the output of small distortion from the output terminal 15.
The optimizing control of the distortion detecting loop 101 in the feedforward amplifier is carried out as follows by controlling the vector regulator 3.
The directional coupler 13 connected to the output of the auxiliary amplifier 12 extracts a part of the signal, the level of which is detected by the level detector 16. The minimum power level of the signal indicates the best canceled state of the signal component, in which the distortion detecting loop 101 is controlled at the optimum state. Therefore, the controller 18 automatically controls the first vector regulator 3 such that the power level detected by the level detector 16 becomes minimum.
Besides, the optimizing control of the distortion canceling loop 102 is carried out as follows by controlling the second vector regulator 11.
The directional coupler 14 installed on the output side of the feedforward amplifier extracts a part of the output signal, and the pilot signal detector 17 detects the pilot signal included in the output signal. The minimum level of the pilot signal indicates the best regulated state of the distortion canceling loop 102. Therefore, the controller 18 automatically controls the second vector regulator 11 such that the pilot signal detected by the pilot signal detector 17 becomes minimum.
Thus, the conventional feedforward amplifier implements the optimum distortion compensation against the ambient temperature variations and deterioration with age by optimally controlling the two loops constituting the feedforward distortion compensation system, that is, the distortion detecting loop 101 and the distortion canceling loop 102.
As conventional feedforward amplifiers, many schemes other than the foregoing method are proposed which carry out the control of the feedforward system by injecting the pilot signal into the loop. All these feedforward amplifiers exploiting the pilot signal have a common problem in that they cannot help outputting the pilot signal from the output terminal. Although the second vector regulator 11 is controlled such that the pilot signal used for controlling the distortion canceling loop 102 is canceled out in principle, the pilot signal is not completely canceled in practice because of the limited control accuracy or nonnegligible control time of the feedforward system. Thus, it is unavoidable that the pilot signal is output from the output terminal 15.
To solve this problem, a filter is often connected to the output terminal of the feedforward amplifier to pass the desired signal and reject the frequency of the pilot signal. However, to achieve the control using the pilot signal at high accuracy, the frequency of the desired signal must be close to that of the pilot signal. Accordingly, it is unavoidable that the filter to separate them becomes large in size and loss, bringing about an increase in size and reduction in efficiency of the amplifier.
In view of this, some schemes are proposed that control the feedforward distortion compensation system without utilizing the pilot signal.
FIG. 2 is a block diagram showing another configuration of the feedforward amplifier disclosed in Japanese patent application publication No. 7-77330. In FIG. 2, the same or like portions to those of FIG. 1 are designated by the same reference numerals, and the description thereof is omitted here.
This feedforward amplifier lacks the pilot signal generator 8 of FIG. 1. It supplies part of the output signal extracted by the directional coupler 14 to the distortion detector 19 for detecting the distortion of the output signal to control the feedforward system in such a manner that the distortion becomes minimum.
This configuration has the following problem.
Specifically, it is very difficult for the feedforward amplifier to carry out the control by detecting the distortion component of its output signal because the distortion component is usually much smaller than the signal component by a factor from 50 dB to 60 dB. Therefore, the distortion detector 19 cannot be realized in practice, or even if it is realized, its circuit configuration will be complicated, resulting in an increase in its size and cost.
Another conventional feedforward amplifier without using the pilot signal is disclosed in Japanese patent application laid-open No. 7-336153. FIG. 3 is a block diagram showing a configuration of the feedforward amplifier. In FIG. 3, the same or like portions to those of FIG. 2 are designated by the same reference numerals, and the description thereof is omitted here. In FIG. 3, the reference numeral 20 designates a controller for controlling the first vector regulator 3; 21 designates a level detector; 22 designates a signal suppressor; 23 designates a controller for controlling the second vector regulator 11; 24 designates a delay circuit; and 25 designates a splitter. The reference numeral 103 designates a distortion detecting loop that comprises the splitters 2 and 25, the first vector regulator 3, the main amplifier 4, the delay circuit 5 and the splitter/combiner 6. FIG. 4 is a block diagram showing a configuration of the signal suppressor 22 as shown in FIG. 3. In FIG. 4, the reference numeral 201 designates a vector regulator, 202 designates a splitter/combiner, 203 designates a delay circuit, 204 designates an amplifier, and 205 designates a level detector.
The feedforward amplifier detects the distortion component signal extracted by the directional coupler 13 by the level detector 21, and controls the first vector regulator 3 by the controller 20 such that the power level of the distortion component signal becomes minimum, thereby carrying out the optimum control of the distortion detecting loop 103. Although the directional coupler 13 is installed before the second vector regulator 11 in FIG. 3, it can be provided after the auxiliary amplifier 12 as in FIG. 1 because the scheme of the optimizing control of the distortion detecting loop 103 is the same as that of FIG. 1.
In FIG. 3, the pilot signal generator 8 as shown in FIG. 1 is not installed. Instead, the splitter 25 provided on the input side of the feedforward amplifier extracts a part of the input signal, and supplies it to the signal suppressor 22 via the delay circuit 24. In addition, the directional coupler 14 on the output side of the feedforward amplifier extracts a part of the output signal, and supplies it to the signal suppressor 22.
The signal suppressor 22 has an internal configuration as shown in FIG. 4. The input signal and output signal of the feedforward amplifier supplied to the signal suppressor 22 are combined by the signal suppressor 22 with the internal configuration including the multi-staged vector regulators 201, splitter/combiners 202 and delay circuits 203. Using the multi-stage internal configuration of the signal suppressor can cancel out the signal component by a factor of 50 dB to 60 dB, leaving the distortion component included in the feedforward amplifier. The distortion component is amplified by the amplifier 204, and detected by the level detector 205. The controller 23 controls the second vector regulator 11 such that the power level of the distortion component is reduced, thereby carrying out the optimizing control of the distortion canceling loop 102.
The conventional feedforward amplifier has a problem of increasing size and complexity because it employs the signal suppressor 22 including the multi-staged splitter/combiners 202, vector regulators 201 and delay circuit 203. In addition, it has a problem in that the adjustment is tedious of the many vector regulators 201 and delay circuits 203 included in the signal suppressor 22.
For example, even the slightest variations in the amplification frequency involved in the change of the channels to be amplified by the feedforward amplifier presents a problem of requiring readjustment of all the vector regulators or all the delay circuits of the signal suppressor 22.
In summary, the conventional feedforward amplifiers with the foregoing configurations have the following problems. First, the feedforward amplifiers that control their feedforward system by injecting the pilot signal have a problem of outputting the residual pilot signal resulting from the control process from the output terminal.
Installing the output filter to eliminate the pilot signal presents another problem of increasing the size and reducing the efficiency of the amplifier because of the large size and loss of the output filter.
As for the configuration as shown in FIG. 2 without the pilot signal generator 8, which controls the feedforward system in such a manner that the distortion becomes minimum by detecting the distortion of the output signal, it is difficult to detect the distortion signal smaller than the signal component by a factor of 50 dB to 60 dB to carry out the control. Thus, it presents a problem in that the distortion detector cannot be implemented in practice, or that even if it can be implemented, its configuration will become complicated, large and expensive.
As for the feedforward amplifier as shown in FIGS. 3 and 4, it presents a problem of increasing its size and complexity because of the signal suppressor 22 with the multi-stage configuration.
In addition, since the slightest variations in the amplification frequency requires the readjustment of all the vector regulators and delay circuits of the signal suppressor 22, it has a problem of requiring complicated adjustment in actual operation.
The present invention is implemented to solve the foregoing problems. Therefore, it is an object of the present invention to provide a feedforward amplifier that can implement favorable distortion characteristic unaffected by the variations in the ambient temperature or deterioration with age, and that has a small size and high efficiency, and can cope with the frequency changes with ease.
According to a first aspect of the present invention, there is provided a feedforward amplifier that includes a distortion detecting loop having a first vector regulator, and a distortion canceling loop having a second vector regulator, and that carries out feedforward distortion compensation, the feedforward amplifier comprising: a directional coupler for extracting a part of an output signal; a first splitter for extracting a part of an input signal; a delay circuit for delaying the input signal extracted by the first splitter; a combiner for combining the input signal delayed by the delay circuit with the output signal extracted by the directional coupler; a local oscillator for generating a signal of a prescribed frequency; a frequency converter for down-converting an output of the combiner to a low frequency using the signal generated by the local oscillator; a first filter for passing a distortion component and for rejecting a signal component of an output of the frequency converter; a distortion component detector for measuring the distortion component output from the first filter; and a second vector regulator controller for controlling the second vector regulator of the distortion canceling loop such that the distortion component measured by the distortion component detector becomes minimum.
According to this, the feedforward amplifier can obviate the need for employing a circuit configuration operating at a high frequency in the control for minimizing the distortion component of the second vector regulator in the distortion canceling loop. This offers an advantage of being able to facilitate implementing the favorable distortion characteristic resistant to variations in the ambient temperature or deterioration with age, to facilitate reduction in size and increase in efficiency, and to cope with the frequency change of the input signal by varying the local oscillation frequency used for the frequency conversion that converts the output of the combiner to the low frequency by the frequency converter.
Here, the feedforward amplifier can further comprise: a third vector regulator interposed between the delay circuit and the combiner for changing pass amplitude and pass phase of an output of the delay circuit; a second splitter interposed between the combiner and the first filter for dividing a signal supplied to it; a second filter for passing a signal component and for rejecting a distortion component of a signal delivered by the second splitter; a signal component detector for measuring the signal component output from the second filter; and a third vector regulator controller for controlling the third vector regulator such that the signal component measured by the signal component detector becomes minimum.
According to this, the feedforward amplifier can obviate the need for the distortion component detector and signal component detector to measure the distortion component and signal component at the radio frequency. This offers an advantage of being able to improve the detection accuracy, and to cancel out the signal component without failure at high accuracy using the input signal and output signal in spite of the variations in the circuit characteristic due to the deterioration with age or ambient temperature variations, thereby implementing good feedforward distortion compensation.
The feedforward amplifier can further comprise: a third vector regulator interposed between the delay circuit and the combiner for changing pass amplitude and pass phase of an output of the delay circuit; a second splitter interposed between the combiner and the first filter for dividing a signal supplied to it; a signal component detector for measuring a signal component delivered by the second splitter; and a third vector regulator controller for controlling the third vector regulator such that the signal component measured by the signal component detector becomes minimum.
According to this, the feedforward amplifier can obviate the second filter for passing the signal component and for rejecting the distortion component of the first low frequency signal distributed by the second splitter, which offers an advantage of being able to implement the feedforward amplifier with the reduced size and cost by an amount of removing the second filter.
The second splitter can be interposed between the combiner and the frequency converter.
According to this, the feedforward amplifier can detect in the radio frequency band the output power obtained by combining the output of the third vector regulator and the part of the output signal extracted by the directional coupler. It offers an advantage of being able to implement the cancellation of the signal component by the combining at practical accuracy, and to carry out the control of the feedforward distortion compensation system at high accuracy.
The distortion detecting loop can comprise an input side splitter for dividing the input signal, and a main amplifier for amplifying a first part of the input signal divided by the input side splitter, wherein the first splitter can further divide a second part of the input signal divided by the input side splitter.
According to this, the feedforward amplifier can obviate the need for installing a splitter on the main path of the input signal from the input terminal of the feedforward amplifier to the splitter/combiner via the main amplifier, that is, on the path of the signal constituting the major part of the output signal of the feedforward amplifier. Thus, it offers an advantage of being able to prevent the reduction in the total gain of the amplifier due to the loss of the splitter, and to implement a favorable distortion characteristic resistant to the variations in the ambient temperature or deterioration with age.
The distortion detecting loop can comprise an input side splitter for dividing the input signal, a main amplifier for amplifying a first part of the input signal divided by the input side splitter, and an intra-distortion-detecting-loop delay circuit for delaying a second part of the input signal divided by the input side splitter, wherein the first splitter can be interposed into a path on an output side of the intra-distortion-detecting-loop delay circuit.
According to this, the feedforward amplifier can utilize the delay circuit in the distortion detecting loop as a part of the delay circuit for delaying the input signal, which is installed on the path of the input signal to be combined with the output signal. Thus, it offers an advantage of being able to miniaturize the delay circuit by an amount corresponding to the delay the input signal undergoes through the delay circuit in the distortion detecting loop, thereby reducing the total size of the amplifier.
According to a second aspect of the present invention, there is provided a feedforward amplifier that includes a distortion detecting loop having a first vector regulator, and a distortion canceling loop having a second vector regulator, and that carries out feedforward distortion compensation, the feedforward amplifier comprising: a first splitter for extracting a part of an input signal; a delay circuit for delaying the input signal extracted by the first splitter; a third vector regulator for changing pass amplitude and pass phase of an output of the delay circuit; a local oscillator for generating a signal of a prescribed frequency; a first frequency converter for down-converting an output of the third vector regulator to a low frequency using the signal generated by the local oscillator; a directional coupler for extracting a part of an output signal; a second frequency converter for down-converting an output of the directional coupler to a low frequency using the signal generated by the local oscillator; a combiner for combining an output of the first frequency converter and an output of the second frequency converter; a second splitter for dividing an output of the combiner; a first filter for passing a distortion component and for rejecting a signal component of a first output of the second splitter; a distortion component detector for measuring the distortion component output from the first filter; a second vector regulator controller for controlling the second vector regulator of the distortion canceling loop such that the distortion component measured by the distortion component detector becomes minimum; a second filter for passing a signal component and for rejecting a distortion component of a second output of the second splitter; a signal component detector for measuring the signal component output from the second filter; and a third vector regulator controller for controlling the third vector regulator such that the signal component measured by the signal component detector becomes minimum.
According to this, the feedforward amplifier can obviate the need for employing radio frequency connecting wire as the connecting wire on the input side of the combiner that combines the output of the third vector regulator and the output signal extracted by the directional coupler, thereby miniaturizing the amplifier. In addition, the feedforward amplifier can utilize a low frequency circuit configuration after combining the output of the third vector regulator and the output signal extracted by the directional coupler. Thus, it offers an advantage of being able facilitate reducing the size and cost of the amplifier.
According to a third aspect of the present invention, there is provided a feedforward amplifier that includes a distortion detecting loop having a first vector regulator, and a distortion canceling loop having a second vector regulator, and that carries out feedforward distortion compensation, the feedforward amplifier comprising: a first splitter for extracting a part of an input signal; a local oscillator for generating a signal of a prescribed frequency; a first frequency converter for down-converting the input signal extracted by the first splitter to a low frequency using the signal generated by the local oscillator; a delay circuit for delaying an output signal of the first frequency converter; a third vector regulator for changing pass amplitude and pass phase of an output of the delay circuit; a directional coupler for extracting a part of an output signal; a second frequency converter for down-converting an output of the directional coupler to a low frequency using the signal generated by the local oscillator; a combiner for combining an output of the second frequency converter and a signal passing through the third vector regulator; a second splitter for dividing an output of the combiner; a first filter for passing a distortion component and for rejecting a signal component of a first output of the second splitter; a distortion component detector for measuring the distortion component output from the first filter; a second vector regulator controller for controlling the second vector regulator of the distortion canceling loop such that the distortion component measured by the distortion component detector becomes minimum; a second filter for passing a signal component and for rejecting a distortion component of a second output of the second splitter; a signal component detector for measuring the signal component output from the second filter; and a third vector regulator controller for controlling the third vector regulator such that the signal component measured by the signal component detector becomes minimum.
According to this, the feedforward amplifier can configure all the circuit components after the frequency conversion by using the low frequency components. Thus, it offers an advantage of being able to facilitate reducing the size and cost of the amplifier.